Thank you for taking a look into this! I had suspected that I should be focusing solely on WP hooks for this, and I have also been unsuccessful at finding the right one.
Since there doesn’t seem to be anything identifiable about a specific “Add Media” button, I should find a hook where the $_SERVER['HTTP_REFERER'] will point to the form’s page plus where I can access the attachment’s ID after it has been uploaded.
I’ve tried the ‘add_attachment‘ action but the referer is always media-new.php, then the ‘wp_handle_upload‘ filter doesn’t seem to provide a way to access the attachment’s ID, and ‘wp_handle_upload_prefilter‘ fires before the upload happens so no luck…
P.S.: Also many thanks for the aforementioned Image field solution. It was very useful for my projects.
